1) Field of the Invention
The present invention relates to a hot-wire air flow meter and to an internal combustion engine provided with such a meter. More particularly, the invention relates to a hot-wire air flow meter suitable as an air flow meter which constitutes the intake system of an automobile internal combustion engine and detects the amount of intake air thereof so as to control the amount of fuel injected.
2) Description of Related Art
In a conventional hot-wire air flow meter, a main air flow passage has a sub-passage disposed in a central portion thereof and a hot-wire element is provided in the sub-passage, as disclosed in Jap. Pat. Laid-Open Nos. 50520/1975, 146369/1975 and 69021/1980. In a hot-wire flow meter having the structure such as that disclosed in Jap. Pat. Laid-Open No. 50520/1975, however, the hot-wire element is defenseless against blow-back due to engine back fire caused when, for example, the timing element is mistimed. As a countermeasure, a structure of protecting the hot-wire element from back fire is disclosed in Jap. Pat. Laid-Open Nos. 146369/1975 and 69021/1980. However, due to the nonlinearilty of a hot-wire element, that is the thermal conductivity is not proportional to the voltage frequency output thereof, a hot-wire element generally has an output characteristic which is lowered in spite of the increased average flow rate when the hot-wire element is placed in a large flow of pulsating air caused by piston movement in an internal combustion engine. All of the above-described prior art air flow meters have the disadvantage that they cannot accurately detect the flow rate of a pulsating air flow.
As disclosed in Jap. Utility Model Laid-Open No. 135127/1981 and Jap. Pat. Laid-Open No. 185118/1985, in some hot-wire air flow meters, a sub-passage with a hot-wire element provided therein is disposed in the main passage such that the fluid resistance of the sub-passage downstream from the hot-wire element is increased as a countermeasure to back firing or in order to accurately detect the pulsating flow and the entrance opening of the sub-passage is parallel to or almost parallel to the main air current. In other words, the dynamic pressure of the back flow which acts on the entrance opening is reduced and the flow going toward the hot-wire element is attenuated, thereby enhancing the resistance to back fire. Since the downstream exit of the sub-passage is directly in line and substantially parallel to the main air flow current, the flow in the sub-passage fluctuates due to the static pressure which is caused by the mixture of the flow from the sub-passage and main passage at this portion. This appears as noise on the hot-wire element. Although high-frequency noise is cut off to a certain degree, the noise caused by the above-described fluctuation becomes a problem in controlling the system when the engine is driven at a low speed. In addition, the known structure, has a long axial length making installation in an automobile difficult and is made of a number of parts making the cost of manufacture high.
There are hot-wire air flow meters in which a sub-passage with a hot-wire element provided therein is disposed outside of the main passage as a countermeasure for back fire and to stabilise the output of the hot-wire element with respect to the intake pulsation, as disclosed in Jap. Pat. Laid-Open Nos. 13557/1972, 109816/1983, 76012/1981 and 28017/1986. The embodiments described in these specifications have the disadvantage that the detection error in the flow rate is increased due to thermal conditions such as the thermal conduction from the engine, the heat of the hot-wire element itself, or the heat of the engine and the rise in temperature in the engine compartment caused by solar radiation, as pointed out in Jap. Pat. Laid-Open No. 76012/1981. That is, since the sub-passage portion is provided in the interior of the body wall which has a large heat capacity and does not have a wide heat transfer area with respect to the air flow, the temperature of the air flow in the sub-passage is influenced by the temperature of the passage wall and the difference in temperature between the air flow in the sub-passage and the air flow in the main passage is increased. This leads to an increase in the error in the measurement of the intake air flow.
Jap. Pat. Laid-Open No. 250260/1985 discloses a structure in which the entrance of the sub-passage has a bell shaped mouth having a large throat area ratio so as to reduce the error in measurement even when the air flow upstream of the entrance of the sub-passage is greatly deflected. This structure, however, does not effect an improvement on the measurement accuracy (the stabilisation of the distribution of the air flow in the main passage and the sub-passage) if the air flow upstream of the entrance of the sub-passage has a large speed distribution and a large pressure distribution. This fact is prominent when the sub-passage is provided eccentrically with the main passage. In addition, this structure has the disadvantage that the flow rate in the sub-passage is increased when there is a large amount of air flow, so that a large amount of dust adheres to the hot-wire element, thereby varying the output characteristic with time.
In the above-described prior art, some have a structure unsuitable for practical use because they do not withstand engine back fire and strong blow-back of the engine and they cannot accurately detect average flow rate of a pulsating flow. Moreover some cannot accurately measure the flow rate when the flow varies due to changes in thermal conditions to which the air flow meter is exposed, nor when different shapes of the constituent parts of the intake pipe passage are arranged upstream of the air cleaner, duct, etc.; additionally since the noise of the output of the hot wire element is large, sufficient control of the engine when driven at the optimum ratio is not carried out, thereby obstructing cleaning of the exhaust gas of the engine, reduction in fuel cost, and improvement in operability, etc. Other prior art meters increase the pressure loss in the intake pipe passage and the weight of the system including the engine, thereby obstructing any reduction in fuel cost, and reduce the space in the engine compartment, etc.
It is an object of this invention to provide a hot-wire air flow meter which achieves a reduction in fuel cost of the engine system and occupies less space in the engine compartment, and which is capable of detecting accurately the amount of intake air under various conditions.
It is another object of the present invention to provide an internal combustion engine which is capable of the optimum control of the air fuel ratio by using the above-described hot-wire air flow meter.